1. Field of the Invention
The present invention relates to a testing process for magneto-optical recording for presetting optimum recording power in magneto-optical medium of the domain wall displacement type, and to a magneto-optical recording-reproducing apparatus.
2. Related Background Art
In magneto-optical media, a multi-layer film comprising a magneto-optical layer and a reflection film has heretofore been formed on a transparent substrate through an insulating film. In general, land (or surface) portions and groove portions are used as data recording regions for storing information, and guiding grooves are used for tracking of a light beam, respectively, in magneto-optical recording. Magneto-optical medium capable of conducting high-density recording have been required with the increase in the quantity of information processing in recent years, and the idea to narrow a track pitch to meet such a requirement quickly has been forwarded. Research at a track pitch of 0.7 to 0.9 xcexcm in a land (groove) type medium has been forwarded, and research to obtain a more narrow track pitch by shortening a laser wavelength has been advanced.
With the requirement of high-density recording, magneto-optical medium of the so-called land and groove type, in which grooves are formed as grooves sufficiently wide in width to write information not only into land portions; but also into groove portions, have also been developed. In the case of the land and groove type magneto-optical media, a pitch between land portions or between grooves can be made wider compared with the land (groove) type media, and so the restriction of a servo system for a light beam can be relaxed to form tracks as narrow as 6.4 to 0.7 xcexcm, in terms of a track pitch.
As described above, information is recorded into the groove and land portions in a land and groove recording, and borders between land portions and groove portions are only slope portions of a substrate in which the land portions and groove portions have been formed, searches of formation of deep grooves, and the like are also collectively forwarded to provide longer slope portions.
On the other hand, in order to prevent destruction of data recorded on adjacent tracks, it is necessary to adjust power of a light beam in recording with high accuracy so as to form separately recorded domains of optimum sizes within land portions and groove portions constituting respective tracks. If the recording power is too high, in the case of a land recording, cross writing that a domain stored in the land portion extends beyond a groove to an adjacent land portion so as to destroy information recorded on the adjacent track occurs. In the case of a land and groove recording, cross writing that a domain stored in the land portion (or groove portion) extends beyond a slope portion of a border to an adjacent groove portion (or land portion) so as to destroy information recorded on the adjacent track occurs.
On the other hand, since magneto-optical recording is heat mode recording, the size of a magnetic domain recorded varies due to the influence of the temperatures of a recording apparatus and a medium used even when recording is conducted under the same recording conditions. Therefore, it is necessary to conduct recording while feeding back the influence of the temperatures of the recording apparatus and the medium into the recording conditions. In addition, individual differences between recording apparatuses due to differences in the form of a light spot caused by aberration of an optical head and defocusing, differences in characteristics between laser light sources, and the like, or scattering of the production of recording media, or the like, also greatly affects recording conditions.
As a method for preventing such cross writing, for example, Japanese Patent Application Laid-Open No.10-134354 has proposed a method for determining an optimum quantity of light for recording. According to this publication, a test pattern A is first recorded on an Nth track in a predetermined quantity of light for recording, and test patterns B are separately recorded on an (N+1)th track and an (Nxe2x88x921)th track in such various quantities of light for recording that a part of the test pattern A is sandwiched between them. Among data recorded on the Nth track, a difference between a reproduction signal from the portion of the test pattern A sandwiched between the test patterns B and a reproduction signal from the portion of the test pattern A not sandwiched between the test patterns B is determined every quantity of light for recording of the test patterns B to define the maximum quantity of light for recording from among the quantity of light for recording the test patterns B, in which the difference between the reproduction signal from the portion of the test pattern A sandwiched between the test patterns B and the reproduction signal from the portion of the test pattern A not sandwiched between the test patterns B falls within a prescribed range, as an optimum quantity of light for recording. Here, as a method for determining the difference between the reproduction signals, there is adopted a method in which a difference between the amplitude of the reproduction signal from the portion of the test pattern A sandwiched between the test patterns B and the amplitude of the reproduction signal from the portion of the test pattern A not sandwiched between the test patterns B, or a difference between standard deviation values of the respective signal levels or between error rates of reproduction is determined.
The linear recording density of an optical disk such as a magneto-optical medium greatly depends on the laser wavelength and the numerical aperture of an objective lens of an optical system for reproduction. More specifically, since the laser wavelength xcex and the numerical aperture NA of the objective lens of the optical system for reproduction determine the diameter of a beam waist, the detectable range of the spatial frequency upon reproduction of magnetic domains recorded is limited to about 2NA/xcex. Therefore, in order to achieve higher recording density with a conventional recording medium, it is necessary to shorten the laser wavelength xcex or enlarge the numerical aperture NA of the objective lens in the optical system for reproduction. However, improvements in the laser wavelength xcex and the numerical aperture NA of the objective lens are limited naturally. Therefore, techniques in which the structure and reading method of a recording medium are devised to improve the recording density have been developed.
For example, in Japanese Patent Application Laid-Open No. 6-290496, the present applicants have proposed a signal-reproducing method, in which signals are recorded in a memory layer of a multi-layer film having a displacement layer and the memory layer magnetically connected to each other, and recorded domains of less than the diffraction limit of an optical system are reproduced by displacing domain walls of the recorded domains in the displacement layer without changing record data in the memory layer by utilizing a temperature gradient caused by irradiation of a light beam for heating, and detecting a change in the polarization direction of reflected light of the light beam. According to this method, recorded domains of frequency of less than the diffraction limit of the optical system can be reproduced without decreasing the amplitude of reproduction signals, whereby recording density and transfer speed can be greatly improved. Such a magneto-optical medium will hereinafter be referred to as a domain wall displacement type magneto-optical medium.
Even when such a domain wall displacement type magneto-optical medium is used, research in which a track pitch is narrowed, thereby enhancing a track density in the magneto-optical medium, like the conventional magneto-optical medium, has been forwarded. In this case as well, the problem of cross writing due to formation of narrow tracks, like the conventional magneto-optical medium, or formation of narrow tracks by land and groove recording arises, and so there is the need of presetting recording power or providing an index to recording power in order to prevent cross writing.
When it is attempted to detect cross writing by the conventional method described in Japanese Patent Application Laid-Open No. 10-134354 or the like, in the case where this domain wall displacement type magneto-optical medium is used, however, a change in the amplitude of reproduction signals has scarcely manifested itself at the point of time the cross writing occurs, and so the optimum recording power cannot be preset. FIG. 11 illustrates the results in the case where cross writing of a domain wall displacement type magneto-optical medium was detected by using the conventional method. An axis of abscissa indicates recording power of adjacent tracks {(Nxc2x11)th tracks}, and an axis of ordinate the amplitude, jitter and pulse width of a reproduction signal of an Nth track. As is apparent from FIG. 11, no change in the amplitude of the reproduction signal manifests itself at the point of time the cross writing occurs. The reason for this is that the reproduction principle of the domain wall displacement type magneto-optical medium comprises reproducing a magnetic domain recorded in a displacement layer by momentarily displacing a domain wall of the magnetic domain so that the waveform of the reproduction signal becomes a rectangular reproduction signal, and thus a delicate change in the width of the magnetic domain by cross writing scarcely manifests itself on the level and amplitude of the reproduction signal, unlike the reproduction signals of the conventional magneto-optical media.
In the case where the error rates of reproduction are used to detect the occurrence of cross writing, the error rate of a reproduction signal is generally designed with some allowance for the jitter of the reproduction signal itself in a magneto-optical recording-reproducing apparatus, and so little deterioration of the jitter is not directly reflected on the error rate of reproduction. However, the degree of allowance in the detection system of the apparatus for this jitter varies from apparatus to apparatus, because there are individual differences between jitters of respective reproduction signal-processing systems themselves.
In addition, since the jitter of a reproduction signal-processing system of an apparatus varies with environmental conditions upon reproduction, such as environmental temperature, and individual differences in laser power for reproduction, focus-tracking conditions and the like between different apparatuses affect the signal to noise ratio (S/N) of reproduction signals, and the degree of allowance in the detection system of the apparatus for jitters of magneto-optical reproduction signals varies from apparatus to apparatus. Accordingly, even when no change in error rate by cross writing is recognized in an apparatus, an error rate may be deteriorated in some cases when adjacent tracks to a track recorded in this apparatus are reproduced in another apparatus, so that the cross writing has been unable to be exactly detected. As described above, the application of the conventional presetting method of the optimum recording power for preventing cross writing to the domain wall displacement type magneto-optical medium has involved a problem that recording power at which cross writing occurs cannot be exactly detected for the above-described reasons.
In view of the above-described problems involved in the prior art, it is an object of the present invention to provide a testing process for magneto-optical recording for presetting optimum recording power applicable to domain wall displacement type magneto-optical media, and a magneto-optical recording-reproducing apparatus therefor.
The above object can be achieved by the present invention described below.
According to the present invention, there is thus provided a testing process for recording for presetting an optimum recording power for a domain wall displacement type magneto-optical medium, the process comprising the steps of:
recording a first record pattern on an Nth track on the magneto-optical medium;
separately recording a second record pattern on an (Nxe2x88x921)th track and an (N+1)th track adjacent to both sides of the Nth track with varied recording power;
reproducing a signal on the Nth track so as to detect a change in the pulse width of a reproduction signal;
detecting a recording power, at which cross writing occurs, on the basis of the detected result; and
presetting the optimum recording power on the basis of the recording power at which cross writing occurs.
According to the present invention, there is also provided a magneto-optical recording-reproducing apparatus for recording information on a domain wall displacement type magneto-optical medium, the apparatus comprising:
a light source for emitting a light beam;
a circuit for recording a first record pattern on an Nth track on the medium and separately recording a second record pattern on an (Nxe2x88x921)th track and an (N+1)th track adjacent to both sides of the Nth track with varied recording power while controlling the light source;
a reproduction circuit for detecting the first record pattern by means of the light beam so as to output a reproduction signal;
a circuit for detecting a change in the pulse width of the reproduction signal; and
a circuit for detecting recording power, at which cross writing occurs, on the basis of the detected result and presetting an optimum recording power on the basis of the recording power detected.